A Schottky diode is a semiconductor diode made by a contact of a metal with a semiconductor. Due to the difference in electron energy levels at the metal and semiconductor surface, conduction occurs over an energy barrier. This conduction is voltage and polarity dependent, which gives rise to the current-voltage curve of the diode. Good conduction occurs in the forward polarity as the current rises exponentially with voltage. However, conduction is restricted in the reverse polarity, where only a “leakage” current flows. The “leakage” current is weakly dependent on voltage. Breakdown of the diode occurs at a high reverse voltage, caused by carriers being accelerated in a very high electric field which reaches a sufficient energy level to create an avalanche of electron-hole pairs in the semiconductor.
Schottky diodes can be compared with semiconductor p-n junction diodes (metallurgical junctions between n-type and p-type semiconductor). In the latter devices, conduction is dominated by minority carriers through the metallurgical junction region. By way of contrast, in Schottky diodes the conduction is dominated by majority carriers and occurs mostly by thermionic emission with some carrier diffusion.
The forward voltage drop (which is the voltage required to conduct reasonably well in the forward conduction polarity) tends to be lower for Schottky diodes compared to semiconductor junction diodes. For silicon p-n junction diodes, the forward voltage drop is typically about 700 mV but for Schottky diodes, comprising e.g. titanium silicide on silicon, the forward voltage can be less than about 100 mV.
Being majority carrier devices, Schottky diodes are inherently faster to respond to electrical signals than junction diodes. The time constant for Schottky diodes is smaller than for junction diodes, which also have an associated diffusion capacitance in forward bias. This is not the case in Schottky diodes. There is virtually no delay for the diode to switch from conducting to non-conducting state because there is no p-type to n-type zone which needs to be formed—as is the case for PN junction diodes. Capacitances for these diodes can also be very low, especially if the semiconductor material is very lowly doped. In reverse bias the diode capacitance is inversely proportional to the semiconductor depletion depth (the region in which the carriers are depleted), which is thicker for low doping levels at a given voltage. The superior operational speed of Schottky diodes makes them very useful in circuits and since they can be integrated into modern semiconductor ICs, they can be a more cost effective solution than alternative components.
Schottky diodes are often used in circuits for high speed rectification of RF signals because of their performance as a fast diode (due to a small time constant, conduction by majority carriers and low capacitance). This application area gave rise to the development of the first point contact rectifiers, known as “cat's whiskers” used in pioneering radio equipment at the start of the twentieth century.
Furthermore, Schottky diodes have a wide range of applications as a general electronic component. For example, they can be used in charge pumps to generate larger in-circuit voltages using a lower source voltage supply. Another common circuit application is the use in diode voltage clamps for preventing over-voltage spikes, e.g. on a power supply line.
Schottky diodes can be made of metallic contacts to any semiconductor material. They can be made of a variety of metals or refractory silicides in direct contact with doped silicon. Hence they are compatible with mainstream planar silicon semiconductor processes which are used for integrated circuit manufacture. They can be combined with many other components on a single silicon chip. However, the integration of the best possible Schottky diode within a semiconductor fabrication process is an engineering challenge.
A Schottky diode is usually made of an n-type semiconductor in contact with a metal. In this configuration, the n-type semiconductor is the cathode and the metal is the anode. This structure is suitable for integration in chips because the wafer substrate on mainstream semiconductor processes is normally a p-type single crystal silicon substrate. Thus the cathode is junction isolated from the wafer substrate and the anode and cathode are available for connection into circuits. It is also possible to form Schottky diodes using p-type semiconductor connected with a metal but this type is rarely used.
Problems are often associated with the manufacture of Schottky diodes. One of these arises from the need for the semiconductor material to be very lightly doped. This is desired to ensure that the junction gives a good current-voltage response and also to keep the reverse bias parasitic capacitance low. However the lightly doped semiconductor has a relatively large series resistance. Hence the maximum current flow can be restricted by the series resistance. Further, self-heating can occur because of the semiconductor resistance, which may cause some thermal instability.
In order to keep the series resistance of the diode low, an appropriate doping and layout style should be chosen. In practice this means that the diode is arranged in alternate stripes of anode and cathode connections. Such an arrangement keeps the resistive effects small because the resistance varies inversely with the width and is proportional to the length (i.e. the pitch between stripes in this case). The cathode regions may have extra doping to reduce ohmic connection resistances. Connections to the cathode are made by heavily doping the semiconductor to a degenerate level and then making a metal connection to the degenerately doped semiconductor. In this case it forms an ohmic (resistive) connection rather than a Schottky (diode) connection. This is because the heavily doped semiconductor behaves like a metal. Metal to metal connections are ohmic in their electrical behaviour, and the number of contacts to the anode and cathode are maximized to reduce the series resistance of these connections. Electrical connecting wires to the rest of the circuit are attached to these metal contacts.
Further improvements within the semiconductor can be achieved by engineering a connection to the cathode which is diffused underneath the lowly doped semiconductor. This may not be absolutely necessary but could be used to minimize parasitic series resistance. A doped n-type region can be diffused vertically to connect the cathode metal and degenerately doped, n+ regions to the buried n-type layer. This can further reduce parasitic series resistance, at the expense of process complexity.
Another problem associated with Schottky diodes is breakdown voltage. This is dependent on the semiconductor doping (lower doping gives a higher breakdown voltage). With a high reverse bias the diode will start to conduct abruptly at the breakdown voltage. This is generally many volts (e.g. 30V), but tends to be somewhat lower for Schottky diodes than for PN junction diodes using a similar semiconductor doping. The edge of the diode is the weakest area in terms of breakdown. The vertical edge sharpness enhances the electric field and so breakdown may be most likely to occur at the edge. Further, the edges are less ideal in their material structure than central regions since they comprise more surface states and traps. This can reduce the breakdown voltage and early breakdown may occur. Before a sharper breakdown, a “soft” breakdown may also occur when the edge related leakage current escalates rapidly at moderate voltages.
However one of the major problems with Schottky diodes is the relatively high reverse bias leakage compared to PN junction diodes. Diode leakage is caused by the generation of extra carriers within the diode metal to semiconductor interface at reverse bias. The quality of the metal to semiconductor interface is critical in determining the reverse leakage. Leakages can be very high, in particular at the edges of a Schottky diode because the edge is usually the most defective region. Further, reverse breakdown voltages tend to be lower at the edges of the Schottky diode due to electric field enhancement at the sharp edge.
One technique of addressing the leakage issues with Schottky diodes involves insetting the metal interface into the semiconductor active area and to add a doped diffused semiconductor ring around the edge of the diode. An example of this technique is illustrated in FIG. 1. The components of the Schottky diode shown in FIG. 1 are as follows:    1. Metallization connection wiring layer; e.g. aluminum copper alloy >300 nm thickness    3. Field isolation dielectric layer; e.g. LOCOS or STI silicon dioxide >200 nm thick    4. P-type semiconductor diffusion; lightly doped; under LOCOS or STI    6. Lightly doped N-type semiconductor cathode; deep n-well    8. Metal Silicide layer, e.g. titanium silicide, used to connect to the semiconductor; used as the schottky diode anode metal node and also formed on top of the guard ring (18) and cathode connection.    9. Metal connection contact plug, e.g. tungsten plug    10. Dielectric layer; isolates wiring layer from the device    11. Highly doped N-type semiconductor for ohmic connection to cathode of diode    14. Lightly doped P-type semiconductor substrate, e.g. silicon single crystal wafer    18. Moderately doped P-type semiconductor diffusion ring termination (guard ring) at the edge of the anode silicide layer
For metal connections to n-type silicon (such as n-type diffusion 6) the additional ring 18 is p-type. This p-type ring 18 forms a PN junction diode at the edge of the component and prevents the very high Schottky diode leakage which would otherwise be seen at the edge.